1. Field of the Invention
This invention relates to the control of corrosion and corrosion products in an olefins production plant that employs a hydrocarbon cracking process such as steam cracking in a pyrolysis furnace.
2. Description of the Prior Art
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes.
This process is carried out in a pyrolysis furnace (steam cracker) at pressures in the reaction zone of from about 10 psig to about 30 psig. Pyrolysis furnaces have internally thereof a convection section and a radiant section. Preheating is accomplished in the convection section, while cracking occurs in the radiant section.
Basically, a hydrocarbon feedstock such as naphtha, gas oil or other fractions of whole crude oil, is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. The steam/hydrocarbon mixture is preheated in the convection section of the cracking furnace to from about 900° F. to about 1,000° F., then enters the reaction zone (the radiant section of a cracking furnace) where it is very quickly heated to a hydrocarbon cracking temperature in the range of from about 1,450° F. to about 1,550° F.
After severe cracking, the effluent from the pyrolysis furnace contains gaseous hydrocarbons of great variety, e.g., from one to thirty-five carbon atoms per molecule. These gaseous hydrocarbons can be saturated, monounsaturated, and polyunsaturated, and can be aliphatic and/or aromatic. The cracked gas also contains significant amounts of molecular hydrogen.
Thus, conventional steam cracking, as carried out in a commercial olefin production plant (olefin plant), employs a fraction of whole crude oil and totally vaporizes that fraction while thermally cracking same. The cracked product can contain, for example, about 1 weight percent (wt. %) molecular hydrogen, about 10 wt. % methane, about 25 wt. % ethylene, and about 17 wt. % propylene, all wt. % being based on the total weight of the cracked product, with the remainder consisting mostly of other hydrocarbon molecules having from 4 to 35 carbon atoms per molecule.
The cracked product is then processed in the olefin plant to produce, as products of the plant, various separate individual streams of high purity such as molecular hydrogen, ethylene, propylene, mixed hydrocarbons having four carbon atoms per molecule, and pyrolysis gasoline. Each separate individual stream aforesaid is a valuable commercial product in its own right.
Thus, an olefin plant takes a part (fraction) of a whole crude oil stream and, through complex and thorough processing of same, generates a plurality of separate, valuable products therefrom.
In addition to these valuable products, the pyrolysis process can produce small amounts of undesirable byproducts which can cause deleterious effects to the process, plant equipment, or both.
One class of chemicals in this category is acids, both organic and inorganic. For various reasons and purposes, acids are introduced into the process, or are made in the process itself, or otherwise find their way into various process streams within an olefin plant. They can cause corrosion problems since the equipment and piping in an olefin plant is largely composed of carbon steel.
For example, acetic acid can be found in various streams in an olefin plant. The acetic acid reacts with iron in carbon steel pipe and equipment thus forming iron acetate and causing corrosion of the carbon steel. The iron acetate is relatively stable and moves on in the olefin plant without causing further damage to the plant until it encounters some oxygen, for example as found in some pyrolysis gasoline (pygas) streams, particularly mixed pygas and C5 streams from an olefin plant that cracks light hydrocarbon feeds. When the iron acetate encounters molecular oxygen, the oxygen destabilizes the iron acetate and breaks it down to ferric oxide and acetic acid, thereby releasing fresh acetic acid to cause additional corrosion in other parts of the olefin plant that are formed of carbon steel by forming more iron acetate. Such a corrosion-acid regeneration-corrosion cycle can go on in many locations throughout an olefin plant.
Iron acetate, for example, dissolves in either a water phase it encounters, or a hydrocarbon phase, or both phases if present. Thus, it will find oxygen in any phase by which to generate fresh acetic acid for additional iron corrosion.
It is desirable to neutralize acids that are coursing through an olefin plant. This is not an easy task.
Many salts can be made of the acids to neutralize same, but it is difficult to find one that will be stable under the various conditions existing in an olefin plant. For example, ammonia could be used to neutralize the aforesaid acetic acid, but ammonium acetate, although more stable than iron acetate in the presence of oxygen still breaks down under certain thermal conditions or certain acidic conditions to free up fresh acetic acid. Under basic conditions ammonia is generated. None of this is desirable in an olefin plant.
Thus, it is highly desirable to have a means by which acids can be neutralized in a stable manner under the varying conditions found in an olefin plant and that will not adversely affect operations in the olefin plant.